Changing Stella's powers is understandable in my eyes because she only used moon-related powers twice out of the whole series. Changing Layla's powers, however, is a different story. Though, to be honest, I do think Fairy of Waves sounds slightly better than Fairy of Fluids. That could have some "interesting" meanings to some people. You know, those with dirty minds.

These aren't really changes. Stella never really used her moon powers. I think she had one moon-based attack, and that was it. Changing her to the fairy of just the sun makes A LOT of sense. As for Layla/Aisha, they didn't change her powers either. They just needed some better way to say "fairy of liquids," which, like funinheaven said, could have awkward meanings. =\ And "fairy of water" doesn't work because she can control more than just water.